A man in his forties was found injured on the Brighton bypass just after the rush hour yesterday evening.
It is believed that he jumped from the Ditchling Road bridge which crosses the A27 to Coldean Lane.
The 47-year-old local man had suffered serious head injuries and was taken to the Royal Sussex County Hospital in Eastern Road, Brighton, for treatment.
He remains in a critical condition.
It was not immediately clear how he had sustained the injuries before he was found on the westbound carriageway just before the Hollingbury slip road at about 6.30pm.
The A27 was closed for three hours between Falmer and Hollingbury while Sussex Police investigated the incident.
Officers investigated whether he had been struck by a car or other vehicle which failed to stop, whether he had been thrown from a moving vehicle or whether he had jumped from the Ditchling Road bridge on to the bypass.
Although officers are not now treating the incident as suspicious, anyone with information is asked to call 0845 6070999.
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